java - Restricting JTextField input to Integers

Question

This question already has answers here:

Answer 1

Do not use a KeyListener for this as you'll miss much including pasting of text. Also a KeyListener is a very low-level construct and as such, should be avoided in Swing applications.

The solution has been described many times on SO: Use a DocumentFilter. There are several examples of this on this site, some written by me.

For example: using-documentfilter-filterbypass

Also for tutorial help, please look at: Implementing a DocumentFilter.

Edit

For instance:

import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.Document;
import javax.swing.text.DocumentFilter;
import javax.swing.text.PlainDocument;

public class DocFilter {
   public static void main(String[] args) {
      JTextField textField = new JTextField(10);

      JPanel panel = new JPanel();
      panel.add(textField);

      PlainDocument doc = (PlainDocument) textField.getDocument();
      doc.setDocumentFilter(new MyIntFilter());


      JOptionPane.showMessageDialog(null, panel);
   }
}

class MyIntFilter extends DocumentFilter {
   @Override
   public void insertString(FilterBypass fb, int offset, String string,
         AttributeSet attr) throws BadLocationException {

      Document doc = fb.getDocument();
      StringBuilder sb = new StringBuilder();
      sb.append(doc.getText(0, doc.getLength()));
      sb.insert(offset, string);

      if (test(sb.toString())) {
         super.insertString(fb, offset, string, attr);
      } else {
         // warn the user and don't allow the insert
      }
   }

   private boolean test(String text) {
      try {
         Integer.parseInt(text);
         return true;
      } catch (NumberFormatException e) {
         return false;
      }
   }

   @Override
   public void replace(FilterBypass fb, int offset, int length, String text,
         AttributeSet attrs) throws BadLocationException {

      Document doc = fb.getDocument();
      StringBuilder sb = new StringBuilder();
      sb.append(doc.getText(0, doc.getLength()));
      sb.replace(offset, offset + length, text);

      if (test(sb.toString())) {
         super.replace(fb, offset, length, text, attrs);
      } else {
         // warn the user and don't allow the insert
      }

   }

   @Override
   public void remove(FilterBypass fb, int offset, int length)
         throws BadLocationException {
      Document doc = fb.getDocument();
      StringBuilder sb = new StringBuilder();
      sb.append(doc.getText(0, doc.getLength()));
      sb.delete(offset, offset + length);

      if (test(sb.toString())) {
         super.remove(fb, offset, length);
      } else {
         // warn the user and don't allow the insert
      }

   }
}

Why is this important?

• What if the user uses copy and paste to insert data into the text component? A KeyListener can miss this?
• You appear to be desiring to check that the data can represent an int. What if they enter numeric data that doesn't fit?
• What if you want to allow the user to later enter double data? In scientific notation?